The present invention relates to data time compression devices and decompression devices.
Data compression consists in generating, from a signal formed by a continuous sequence of data at rate F.sub.n, a sequence of periodic packages of N data of a duration t.sub.bc, and for which the data rate F.sub.p is greater than F.sub.n.
Such compression is achieved, for example, by filling a memory at rate F.sub.n and by emptying this memory at rate F.sub.p every t.sub.b, where t.sub.b is the duration of a sequence of N data before compression. Decompression takes place similarly by means of a memory whose filling and emptying rates are reversed.
In conventional devices, the two clock signals giving the rates F.sub.n and F.sub.p are unchanged during the whole period of compression of the continuous flow of data. Now, at the beginning of each data package, the clock signals must be in phase, which places a restraint on the value t.sub.b ; it must be chosen with respect to the desired compression rate.
In fact, the values of periods T.sub.n and T.sub.p, corresponding respectively to rates F.sub.n and F.sub.p, are bound together by the relationship T.sub.p =(p/n).times.T.sub.n (p and n positive integers, prime in relation to each other).
This latter relationship must be confirmed so that there exists a phase correspondance between F.sub.n and F.sub.p at the risk of not being able to restore at reception a continuous data flow at rate F.sub.n.
The compression of a package of N data (N=t.sub.b /T.sub.n) produces a "blank" of duration .tau. during which no data is transmitted. This duration .tau. corresponds to the difference between the duration t.sub.b of the package of N data before compression and the duration t.sub.bc of the package after compression.
Assuming u=T.sub.n /n, which entails t.sub.b =N.n.u and t.sub.bc =N.p.u, it may be deduced that .tau.=N(n-p)u.
I, as in the case of conventional compressers, the phase of the clock signal giving the rate F.sub.p is not modified at the end of the existing "blank", it is necessary to choose the duration of the "blank" equal to a whole number of period T.sub.p to ensure synchronization of the two clock signals at the beginning of each data package.
This is only possible when N(n-p)=kp, i.e. N=k'p (k positive integer, k'=k/(n-p).
Thus the number N cannot be any number, it depends on p. Which, for a given compression rate, quantifies the duration t.sub.b of the packages of N data among a limited set of possible values.
The present invention aims at palliating this disadvantage of conventional compressors by allowing operation of the compressors whatever the duration of the data packages for a fixed compression rate.
In the rest of the text and in our claims, a signal formed by a continuous flow of data at repetition frequency F.sub.n will be called "signal to be compressed" when this is the input signal of the compression device of the invention, and "decompressed" signal when it is a question of the output signal of the decompression device of the invention. Similarly, the signal formed by a sequence of periodic data packages, of duration t.sub.bc, and whose data rate is F.sub.p will be called "compressed" signal. The packages of N data of a "compressed" signal will be called "compressed" packages; those of a signal "to be compressed" will be called packages "to be compressed".